Library Statistics - 7. Purchase orders

[JakobMiesner]

Library Statistics

This ticket is part of a larger issue to implement KPIs that measure certain aspects of the library.
An RFC regarding the larger issue can be found here.
This issue implements one of those KPIs.

The KPIs

7. Purchase orders:
   1. Average Waiting time: waiting time of purchase orders / number of purchase orders that were received
   2. Average ordering time: ordering time of purchase orders / number of purchase orders that were received

A detailed plan for the implementation can be found in the RFC.